Considerable research has been conducted in the past to devise inorganic glasses exhibiting low transition temperatures (Tg), thereby enabling melting and forming operations to be carried out at low temperatures with consequent savings in energy costs. More recently, it has been recognized that glasses demonstrating low transition temperatures are potentially useful materials for a host of applications including low temperature sealing glasses and glass-organic polymer composites. A very recent development disclosed in U.S. Pat. No. 5,043,369 (Bahn et al.) involves the production of glass-organic polymer alloys. Those alloys are prepared from a glass and a thermoplastic or thermosetting polymer having compatible working temperatures. Thus, the glass and the polymer are combined at the working temperature to form an intimate mixture; that is, the glass and polymer are in a sufficiently fluid state to be blended together to form a body displaying an essentially uniform, fine-grained microstructure in which, desirably, there is at least partial miscibility and/or a reaction between the glass and the polymer to promote adhesion and bonding therebetween. An article is shaped from the blend and then cooled to room temperature. Such articles exhibit chemical and physical properties comprising a composite of those demonstrated by the particular glass and polymer. For example, the alloys frequently display a combination of high surface hardness, high stiffness, and high toughness.
Glasses having base compositions within the general zinc phosphate system have been found to be especially suitable for the glass component of glass-polymer alloys. Several illustrations of recent research in that composition system are reported below.
U.S. Pat. No. 4,940,677 (Beall et al.) discloses glasses exhibiting transition temperatures below 450.degree. C., preferably below 350.degree. C., consisting essentially, in mole percent, of at least 65% total of 23-55% ZnO, 28-40% P.sub.2 O.sub.5, and 10-35% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions selected from the group of 0-25% Li.sub.2 O, 0-25% Na.sub.2 O, and 0-25% K.sub.2 O, and up to 35% total of optional constituents in the indicated proportions selected from the group of 0-6% Al.sub.2 O.sub.3, 0-8% B.sub.2 O.sub.3, 0-8% Al.sub.2 O.sub.3 +B.sub.2 O.sub.3, 0-15% Cu.sub.2 O, 0-5% F, 0-35% PbO, 0-35% SnO, 0-35% PbO+SnO, 0-5% ZrO.sub.2, 0-4% SiO.sub.2, and 0-15% MgO+CaO+SrO+BaO+MnO, consisting of 0-10% MgO, 0-10% CaO, 0-10% SrO, 0-12% BaO, and 0-10% MnO.
U.S. Pat. No. 4,996,172 (Beall et al.) describes glasses demonstrating transition temperatures below 350.degree. C. consisting essentially, in mole percent, of 30-55% ZnO, 28-45% P.sub.2 O.sub.5, 10-35% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions of 0-25% Li.sub.2 O, 0-25% Na.sub.2 O, and 0-25% K.sub.2 O, 0-4% Al.sub.2 O.sub.3, 0-10% total of at least one divalent metal oxide selected from the group of CaO, MgO, and MnO, 0-15% CuO, 0-35% PbO, 0-35% SnO, 0-35% SnO+PbO, and 0.5-5% total of Y.sub.2 O.sub.3 and/or at least one rare earth metal oxide of the lanthanide series.
U.S. Pat. No. 5,021,366 (Aitken) is directed to glasses having annealing points between 300.degree.-340.degree. C. which are fluorine-free and consist essentially, in mole percent, of
__________________________________________________________________________ Li.sub.2 O 5-10 P.sub.2 O.sub.5 30-36 Na.sub.2 O 5-15 Al.sub.2 O.sub.3 0-5 K.sub.2 O 0-6 CeO.sub.2 0-2 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 15-25 SnO 0-20 ZnO 10-33 PbO 0-20 CaO 0-20 Sb.sub.2 O.sub.3 0-12 SrO 0-20 Bi.sub.2 O.sub.3 0-6 BaO 0-20 SnO + PbO + Sb.sub.2 O.sub.3 + Bi.sub.2 O.sub.3 0-20. CaO + SrO + BaO 12-25 __________________________________________________________________________
U.S. Pat. No. 5,071,795 (Beall et al.) illustrates glasses exhibiting transition temperatures no higher than 350.degree. C. consisting essentially, in mole percent, of
______________________________________ Li.sub.2 O 0-25 ZnO 25-50 Na.sub.2 O 5-20 Al.sub.2 O.sub.3 0-3 K.sub.2 O 0-12 P.sub.2 O.sub.5 25-37 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 15-35 SrO 0-10 ______________________________________
to which are included 0.5-8% Cl and 0-5% F as analyzed in weight percent. Up to 10% Cu.sub.2 O, up to 3% SiO.sub.2, and up to 8% total of at least one alkaline earth metal oxide may be included.
U.S. Pat. No. 5,122,484 (Beall et al.) is drawn to glasses having transition temperatures no higher than about 425.degree. C. which were specifically designed for use as sealing frits in television picture tube applications. The glass compositions were PbO-free and consisted essentially, in weight percent, of:
__________________________________________________________________________ Li.sub.2 O 0.75-5 SnO.sub.2 0-10 Na.sub.2 O 2-10 MoO.sub.3 0-10 K.sub.2 O 2-10 WO.sub.3 0-10 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 5-25 MoO.sub.3 + WO.sub.3 2-25 ZnO 29-42 Cl 0-8 (analyzed) P.sub.2 O.sub.5 38.50 SnO.sub.2 + MoO.sub.3 + WO.sub.3 + Cl 2-25. Al.sub.2 O.sub.3 0-5 __________________________________________________________________________
One inherent drawback of phosphate-based glass compositions having low transition temperatures is their reduced resistance to attack by water and mild solutions of acids and bases, when compared to silicate-based glasses. The above-described zinc phosphate glasses demonstrate relatively excellent resistance to chemical attack, when compared to other phosphate-based glasses. Nevertheless, the search has been continuous to discover new glass compositions manifesting low transition temperatures with even greater chemical durability.
Accordingly, the principal objective of the present invention was to devise glass compositions having transition temperatures below 375.degree. C., preferably between about 300.degree.-350.degree. C., wherein the chemical durability is much improved over that of zinc phosphate-based glasses.